Attached Link: http://www.renderosity.com/mod/gallery/index.php?image_id=1217007&member

Try to reimport  your background.  When it tells you that it will change the dimensions of the preview window, click OK.  The background you selected has no natural light.  It's still okay, but it means that you will have to change the color of some of the lights and possibly reposition a few.

It's what I did for the image in the link - so it can still look natural.